refactor: restructure repository with separate backend and frontend directories

- Move Java backend to backend/ directory
- Create frontend/ directory for TypeScript TUI and future WebUI
- Update .gitignore for Node.js and worktrees
- Update README.md with new repository structure
- Copy documentation to backend/

backend/docs/mvp/ddd/08-procurement-bc.md

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+# Procurement BC - Detailliertes Domain Model
+
+**Bounded Context:** Procurement
+**Domain Type:** CORE
+**Verantwortung:** Bedarfsplanung, Bestellwesen, Wareneingangskontrolle, Lieferanten-QM
+
+## Aggregates
+
+### PurchaseOrder (Aggregate Root)
+
+```
+PurchaseOrder
+├── PurchaseOrderId
+├── SupplierId
+├── OrderDate
+├── RequestedDeliveryDate
+├── Status (DRAFT | ORDERED | CONFIRMED | PARTIALLY_RECEIVED | FULLY_RECEIVED | CANCELLED)
+├── OrderedBy (UserId)
+├── BranchId
+├── Items[] (Entity)
+│   ├── ArticleId
+│   ├── OrderedQuantity
+│   ├── ReceivedQuantity - Updated on goods receipt
+│   └── UnitPrice
+├── TotalAmount - Calculated
+└── PaymentTerms
+
+Invariants:
+- Status cannot skip states
+- TotalAmount = SUM(Items.Quantity * Items.UnitPrice)
+- ReceivedQuantity <= OrderedQuantity per item
+- FULLY_RECEIVED when all items received
+```
+
+### GoodsReceipt (Aggregate Root)
+
+```
+GoodsReceipt
+├── GoodsReceiptId
+├── PurchaseOrderId
+├── SupplierId
+├── SupplierDeliveryNote
+├── ReceivedAt
+├── ReceivedBy (UserId)
+├── Items[] (Entity)
+│   ├── ArticleId
+│   ├── OrderedQuantity
+│   ├── ReceivedQuantity
+│   ├── SupplierBatchNumber - CRITICAL for traceability!
+│   ├── ExpiryDate (MHD)
+│   └── QualityStatus (ACCEPTED | REJECTED | PENDING_INSPECTION)
+├── QualityInspectionId - Reference to Quality BC
+├── Documents[] (Entity) - Photos/PDFs
+└── Status (PENDING_INSPECTION | ACCEPTED | REJECTED)
+
+Invariants:
+- Cannot accept without completed quality inspection
+- All items must have SupplierBatchNumber
+- Veterinary certificate required for meat
+- Triggers StockMovement when accepted
+```
+
+### DemandPlan (Aggregate Root)
+
+```
+DemandPlan
+├── DemandPlanId
+├── PlanningPeriod (Week or Month)
+├── BranchId
+├── GeneratedAt
+├── Items[] (Entity)
+│   ├── ArticleId
+│   ├── CurrentStock - Snapshot from Inventory
+│   ├── PlannedConsumption - From ProductionOrders
+│   ├── AverageConsumption - Historical
+│   ├── SuggestedOrderQuantity - Calculated
+│   └── MinimumOrderQuantity - Supplier constraint
+└── Status (DRAFT | APPROVED | ORDERED)
+
+Invariants:
+- SuggestedOrderQuantity = PlannedConsumption - CurrentStock + SafetyStock
+- If < MinimumOrderQuantity, use MinimumOrderQuantity
+```